Experiments in the system KAlSi04-NaAlSi04-SiOj-Fe-0-H indicate that approximately 20 mole per cent of sodium annite [NaFe3AlSiO10(OH)2] can enter into solid solution with potassium annite. This substitution reduces the biotite stability field. With excess alkali feldspar and no free quartz, the following reactions limit the biotite stability:Feldspar(s)+biotite ⇋ feldspar(s)+magnetite+vapor. (1). Feldspar+biotite ⇋ feldspar+fayalite+leucite+vapor. (2). Feldspar+biotite ⇋ feldspar+fayalite+liquid+vapor. (3). Biotite ⇋ feldspar+fayalite+nepheline+vapor. (4). On the QFM buffer (PTotal = 2 Kb), reaction 1 occurs at 685 °C in the sodium free system and at 625 °C when two feldspars are present; on NNO, it occurs at 630 and 590 °C. On the G-CH buffer, reaction 2 is stable at high temperatures (795-830 °Q and potassic compositions (), reaction 3 at intermediate conditions (T = 740-95 °C; ) and reaction 4 at lower temperatures (710-40 °C) and sodic compositions (). In the presence of excess quartz, reaction 1 remains stable below the QFM buffer, but the reaction Feldspars)+biotite+quartz ⇋ feldspars)+fayalite+vapor (5) is stable at higher hydrogen fugacities. On the G-CH buffer, reaction 5 occurs at 610 °C in the sodium free system and at 595 °C when two alkali feldspars are present. On the MW buffer, the temperatures are 585 and 515 °C (M.I.).The experimental data presented suggest that biotite will not be stable in the presence of granitic liquids at total pressures below 4 kb, providing the fluorine and titanium content of the biotite is low. They also suggest that gradients in the a£1S, l0 in a rock could produce variations in the biotite Fe/Fe+Mg ratio and in extreme cases could result in the complete breakdown (decreased ) of biotite or its crystallization (increased ) where previously there was none. © 1969 Oxford University Press.
